Recently, earth environmental problems have drawn attention. It has been thus demanded suppression of consumption of fossil fuels such as petroleum oil, coal or the like for preventing global warming. It has been thus demanded utilization of clean energies without discharging CO2 in various countries. Wind turbine generation is known as one of the clean energies. According to the wind turbine generation, wings called blades are pressed by wind so that the blades are rotated. The rotation is then transmitted to a rate increaser (gear box) through a power transmission axis. The rotation rate is increased by the rate increaser so that the rotation is converted to electric power through a generator.
In the rate increaser, it is used a gear oil for lubricating a gear and for preventing the wearing. Generally, for the gear oil for use in the wind turbine generator, it is used a hydrocarbon-based base oil having a high viscosity and of a mineral oil or of polyolefin base.
Further, recently, it is developed a wind turbine generation system fixed on a sea. The wind turbine generation system on the sea is superior than that on a land in that the generation can be performed on the sea with strong and stabilized wind blown and in that it can be obtained an electric power larger than that obtained by the generation system on the land having the same scale. However, in the case that the gear oil is leaked in the wind turbine generation system on the sea, the leaked gear oil is easily flown into the sea. It is thus demanded that the gear oil has low load on the environment. However, the gear oil of hydrocarbon base with a high viscosity conventionally used has low biodegradability. It has been thus studied a gear oil having high biodegradability.
For example, according to patent document 1, it is disclosed a gear oil composition superior in biodegradability and comprising an acidic amine salt of a phosphoric acid and an ester base oil synthesized by a polyhydric alcohol, a saturated dicarboxylic acid and a saturated monocarboxylic acid.
Further, as the wind generation system is fixed on a place to which wind is easily blown, excellent low-temperature fluidity is also demanded so that the system can be operated in the case that the temperature of the oil is lowered. The excellent stability at a low temperature is not given to the gear oil composition of patent document 1.
As a technique for solving the object relating to the low-temperature fluidity described above, it is disclosed, in patent document 2, a biodegradable gear composition having excellent low-temperature fluidity. The biodegradable composition is composed of ester base oil synthesized by a polyhydric alcohol, a saturated dicarboxylic acid and a saturated monocarboxylic acid, an acidic amine salt of a phosphoric acid and a hindered polyol ester.